A vial is a sealed container for accommodating a pharmaceutical product or the like. The vial can be sealed by fitting a stopper (rubber stopper or the like) made of an elastic material into a mouth portion of the vial after filling a pharmaceutical product or the like inside the vial. Moreover, when an injection solution or a transfusion preparation is accommodated inside the vial, a parenteral injection can be transferred into an injector through an injection needle or the transfusion preparation can be used as it is for transfusion through a spike needle (bottle needle) with the stopper fitted into the mouth portion (namely, without removing the stopper) by piercing the injection needle or the spike needle through the stopper.
The sealing state of a vial needs to be maintained surely by fixing the stopper so as not to come off the mouth portion from the viewpoint of quality assurance and safety of pharmaceutical products and the like. Moreover, with respect to the vial used for parenteral injections or transfusion, it is necessary to prevent as much as possible a situation that the stopper is contaminated at a stage prior to use such as a stage in the distribution process, while a structure by which the stopper can easily be exposed at the time of use is necessary.
Thus, there is proposed a cap to be attached to the mouth portion to fix a stopper fitted into the mouth portion of a vial so that the stopper may not come off the mouth portion. Conventionally, aluminum caps have generally been used as such a cap (vial cap); however, resin caps have also been proposed recently. For example, there is proposed a plastic cap including: a cap main body provided with a window portion at the center portion of a top board; and a lid portion covering the window portion, in which plastic for welding is filled in a hole for welding which is formed in the cap main body, and the cap main body and the lid portion are welded and integrated by the filled plastic for welding (see Patent Literature 1).